Preparation of halopolycarboxy benzene by oxidation of halopolyalkyl benzene



United States Patent PREPARATION OF HALOPOLYCARBOXY BEN- ZENE BY OXIDATION OF HALOPOLYALKYL BENZENE Willis C. Keith, Lansing, and Robert R. Chambers, Homewood, 11]., assignors, by mesne assignments, to Sinclair Research, Inc., New York, N.Y., a corporation of Dela- Ware No Drawing. Filed Oct. 20, 1958, Ser. No. 768,015

5 Claims. (Cl. 260-524) The present invention relates to a process for the preparation of aromatic polycarboxylic acids and more specifically it presents an improved method of oxidizing, in the presence of molecular oxygen and in the liquid phase a benzene compound containing at least one ring halogen and at least two alkyl radicals, to its corresponding halopolycarboxyl benzene compound.

In the past, numerous methods have been attempted to bring about the air oxidation of aromatic compounds to the corresponding aromatic acids, especially xylenes to phthalic acids. These oxidation reactions have generally been carried out in the presence of a catalyst such as cobalt toluate, cobalt naphthenate, etc. The main difficulty with these reactions has been that the ultimate yield of polycarboxylic acids has been low, e.g. about 20% in a one-stage process, with the preponderance of the product being the monocarboxylic acid. In order to go to the dior polycarboxylic acid it is usually necessary to esterify the acid, halogenate the alkyl groups or, alternatively, employ a chemical oxidizing agent such as nitric acid. Recently issued Australian Patent No. 16,742/56 describes a method of oxidizing aromatic compounds to aromatic acids. This patent provides a onestage method for the liquid phase air oxidation of alkyl substituents to polycarboxylic aromatic acids by the use of a metal, e.g. cobalt, and bromine as the oxidation cocatalysts.

An article appearing in Industrial and Engineering Chemistry, vol. 41, No. 11, page 2615, entitled, Oxidation of Aromatic Compounds, is also of interest on this subject and discloses a method for effecting the liquid phase oxidation of alkyl aromatics in the presence of hydrogen bromide. The oxidation process described therein is conducted in the absence of a solvent, and the acid product is substantially contaminated with various impurities such as brominated reaction products and various phenolics. The patent to Rust et al. US. Patent No. 2,415,800 also discloses the oxidation of alkyl aromatics while in the presence of hydrogen bromide. In this patent the oxidation reaction is generally carried out in the vapor phase and in the absence of a solvent. The hydrogen bromide apparently acts as an inhibitor to retard the decomposition of the hydrocarbon feedstock in order to obtain oxidation products having the same number of carbon atoms as the feed.

In accordance with the present invention a polyalkyl and halogen substituted benzene is oxidized in the liquid phase at elevated temperatures, say about 150 to 350 C., and pressure sufiicient to keep the liquid phase with molecular oxygen in the presence of hydrogen bromide and a material which apparently acts as a solvent for the reactants. In many instances the conversion of the feedstock can be 100% or substantially quantitative. The resulting halogen substituted aromatic polycarboxylic acid which is produced in exceptional yields, provides a valuable intermediate having polyfunctional reactive characteristics. Halo-substituted phthalic acids, for example, are valuable starting materials for polyester resins, providing synthetic fibers with a built-in dye-site. Chloroand bromo acids produced by the process of this invention may be hydrolyzed to hydroxy acids which in turn may be'esterified with themselves to form polyester materials, or may be aminated to produce an intermediate for polypeptide production.

The process of the invention is applicable to benzene aromatic compounds containing as sole substituents at least two alkyl substit'uents and one or more halogens, the total alkyl and halogen groups being, of course, no more than six. T he alkyl substituents will generally contain 1-4 carbon atoms, and at least two of the alkyl groups must be non-tertiary, that is, other than t-butyl, since this substituent is not oxidized by the process of this invention. The halogen is a middle halogen chlorine or bromine, preferably chlorine, having an atomic number from 17 to 35.

The following are representative halo alkyl benzene compounds which are capable of being oxidized in accordance With the present invention: the chloroand bromoxylenes, mesitylenes and durenes; ortho-, meta-, or parachloro or bromo methyl ethyl benzene, etc. These compounds can be obtained in any conventional manner, such as by the ring halogenation of xylene, etc., using a halogen carrier, such as an iron catalyst. The halogenated products obtained by this or similar procedures can be separated and purified as by distillation or the crude reaction mixture can be sent directly to the oxidation reaction zone.

In order to obtain the desired yields of polycarboxylated acids in a high degree of purity, it is necessary that the reaction be conducted in a medium which affects the solubilization of the reactants. More particularly, the

oxidation reaction is carried out in the presence of a solvent such as acetic acid and substituted derivatives thereof such as trichloroacetic acid or another organic carboxylic acid inert to the present oxidation conditions. The reaction medium can be either anhydrous or hydrated. No particular advantageous or deleterious results are noticeable, however, by the deletion or inclusion of water in the system. The amount of solvent can vary over a Wide range and is usually provided in amounts sufiicient to solubilize a substantial amount of the feedstock. The amount of solvent will primarily depend upon the choice of solvents and the solubility of the particular feedstock therein. It is not necessary that the solvent be provided in amounts suflicient to solubilize the entire charge of feedstock but enough must be present to put enough of the feedstock into solution to effect the oxidation, with additional amounts of the feedstock being solubilized as the reaction products are formed. Generally, the solvent Will be provided in amounts ranging from about 25 to 2000 percent by weight of the feedstock and preferably about to 500 percent by Weight.

The material which is used to catalyze the oxidation reaction is hydrogen bromide and it can be introduced as such into the system or it can be formed in-situ during the course of the reaction. For instance, I can introduce materials such as free bromine, alkyl bromides, hypobromous acid, etc. into the system and under the reaction conditions they will yield hydrogen bromide. The hydrogen bromide will be provided in amounts sufficient to catalyze the reaction and can vary over a wide range with no particular limits capable of being ascertained. This catalyst is effective in the absence of metals such as cobalt. Generally, the catalyst will be provided in amounts ranging from as low as about 0.0001 mol percent up to as high as about 25.0 mol percent based on the aromatic feedstock. Preferably, however, the hydrogen bromide should be available in amounts ranging from about 0.1 to 0.5 mol percent. The hydrogen bromide is used preferably as an aqueous solution containing about 30-60% 'HBr. The catalyst is advantageously introduced into the reactor as a mixture with the aromatic feed.

A gas containing molecular oxygen is suitable for use Patented Apr. 23, 1963 3 in this invention as the oxidizing gas, preferred gases being air and oxygen-enriched air. The contact time is dependent on the reaction conditions (temperature and pressurey'and may be varied over a wide range, for instance, about 0.01 to hours. Normally the preferred contact time is from about 0.1 to 3 hours. The preferred temperature for the oxidation of aromatic compounds is between about 175 and 250 C. The temperature range included within the scope of this invention is from about 50 to 300 C. The most desirable pressure range is from about 100 to 500 psig. The minimum pressure is the pressure required to maintain the liquid phase, whereas little is gained by employing pressures above about 1000 p.s.1.g.

- By way of illustrating the present invention there are presented below several specific examples of the oxidation procedure and the results obtained thereby. The examples show only the oxidation step and not the halogenation of the aromatic hydrocarbon since this is a wellknown conventional procedure. They are not designed to limit the claims or processing procedures than can be used commercially; the experimental procedure described was used to prevent any trace metal contamination. A 120 ml. glass tube sealed at both ends was used on the inside of .a 300 ml. shaker bomb. The tube contained a small hole in the top side for the induction of oxygencontaining gas and also served in equalizing pressure within the bomb. The starting material, catalyst and solvent were charged to the glass tube. After closing the steel shaker bomb, an oxygen enriched gas (60% O and 40% N was introduced until a pressure of 300 p.s.i.g. was obtained. Heat was applied and the reaction controlled at this temperature until completed. At the end of this time the bomb was placed in cold water and after cooling to about 20C., the contents of the tube were removed.

Example I When two ml. of 2-bromo-p-xylene, 18 g. CH COOH and 0.16 ml. of an aqueous 48% HBr solution were charged to the glass liner, pressured to 300 psig. with the 60% O /40% N mixture, and held at 200 C. for 1 hour, 60% of the aromatic reactant Was converted to 2-bromo-terephthalic acid, having a saponification number of 468.

' Example 11 3 ml. of 2-chloro-p-xylene, 0.16 ml. of aqueous 48% HBr and 18 g. of glacial acetic acid were charged to the glass tube. The oxidation conditions were a temperature of 210 C. and a pressure of O /40% N of 300 p.s.i.g. After reaction for 2 hours and subsequent cooling, 2.6 g. of insoluble product was removed by filtration and washed with water and alcohol. The product was a mixture of 2-chloro-terephthalic acid and 2-chloro-ptoluic acid. Some unreacted starting material was also recovered.

Example Ill 4 grams of trichloro-p-xylene, 0.22 ml. of 48% aqueous HBr and 18 grams of glacial acetic acid were charged into the glass tube. The oxidation conditions were the same as those employed in Example '11. A yield of 3.08 grams of a mixture of trichloro-terephthalic and trichlorotoluic acid was obtained.

It is thus seen that the process of this invention pro vides a simple means for conversion of the poly alkyl substituents of a halo alkyl benzene to carboxyl groups using air or other inexpensive source of molecular oxygen, and a solvent.

We claim:

' l. A method for the preparation of a halogen substituted benzene polycarboxylic acid which consists essentially of oxidizing in the presence of molecular oxygen and in the liquid phase a substituted benzene having as sole substituents at least two non-tertiary alkyl radicals of l to 4 carbon atoms and at least one ring halogen having an atomic number from 17 to 35, at an elevated temperature and a pressure sufficient "to maintain the liquid phase, said oxidation being carried out in the presence of a catalytic amount of hydrogen bromide and an acetic acid solvent.

A method as in claim 1 wherein the solvent is acetic aci 2 3. The method of claim 1 wherein the temperature is about to 350 C.

4. A method as in claim 5 wherein the substituted benzene is a halo-xylene.

5. The method of claim 1 wherein the solvent is acetic acid and the temperature is about 150 to 350 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,421,392 Rust et a1. June 3, 1947 2,480,971 Rust et a1. Sept. 6, 1949 2,833,816 Safi'er et a1. May 6, 1958 

1. A METHOD FOR THE PREPARATION OF A HALOGEN SUBSTITUTED BENZENE POLYCARBOXYLIC ACID WHICH CONSISTS ESSENTIALLY OF OXIDIZING IN THE PRESENCE OF MOLECULAR OXYGEN AND IN THE LIQUID PHASE A SUBSTITUTED BENZENE HAVING AS SOLE SUBSTITUENTS AT LEAST TWO NON-TERTIARY ALKYL RADICALS OF 1 TO 4 CARBON ATOMS AND AT LEAST ONE RING HALOGEN HAVING AN ATOMIC NUMBER FROM 17 TO 35, AT AN ELEVATED TEMPERATURE AND A PRESSURE SUFFICIENT TO MAINTAIN THE LIQUID PHASE, SAID OXIDATION BEING CARRIER OUT IN THE PRESENCE OF A CATALYTIC AMOUNT OF HYDROGEN BROMIDE AND AN ACETIC ACID SOLVENT. 